Nucleation-growth processes and isothermal kinetics of phase transformations in the methylhydrazine monohydrate

Abstract

Phase transformations in the methylhydrazine monohydrate (CH 3NHNH 2 · H 2O) are investigated by differential scanning calorimetry (DSC) over the temperature range 110–300 K. On cooling a glass is formed by quenching the liquid. Depending on the heating rate, two solid phases (metastable and/or stable) with slow phase transformation kinetics are observed. The optimal temperature for nucleation and growth processes have been determined for the two solid phases. The nucleation rate is maximum in the glass transition region ( T g ≅ 160 K) and the temperature growth regions are, respectively, estimated to be 188–202 K for the metastable solid phase and 212–228 K for the stable solid phase. The metastable and the stable solid phases, respectively, melt at 220 and 234 K. The kinetic character of phase transitions is revealed by complementary X-ray measurements at 200 K. A phenomenological model based on Avrami's law is proposed for a better understanding of the growth mechanisms for the two solid phases. On reheating, the glassy state transforms to a metastable liquid. Then, a partial transformation (48%) into the metastable solid phase from this liquid begins at about 188 K. It is followed by the growth of the stable solid phase from the undercooled liquid and the metastable solid phase. The solid-solid transformation seems to be governed by contacts with the stable solid grains.